[3H]-domperidone labels only a single population of receptors which convert from high to low affinity for dopamine in rat brain

The High Affinity Dopamine D2 Receptor Agonist MCL-536: A New Tool for Studying Dopaminergic Contribution to Neurological Disorders

The dopamine D₂ receptor exists in two different states, D₂^high and D_2low; the former is the functional form of the D₂ receptor and associates with intracellular G-proteins. The D₂ agonist [³H]MCL-536 has high affinity for the D₂ receptor (K_d 0.8 nM) and potently displaces the binding of (R-(-)-N-n-propylnorapomorphine (NPA; K_i 0.16 nM) and raclopride (K_i 0.9 nM) in competition binding assays. Here, we further characterize [³H]MCL-536. [³H]MCL-536 was metabolically stable, with about 75% of the compound remaining intact after 1 h incubation with human liver microsomes. Blood-brain barrier penetration in rats was good, attaining at 15 min a % injected dose per gram of wet tissue (%ID/g) of 0.28 in males versus 0.42 in females in the striatum. Specific uptake ratios ([%ID/g striatum]/[%ID/g cerebellum]) were stable in males during the first 60 min and in females up to 15-30 min. The D₂-rich striatum exhibited the highest uptake and slowest washout compared to D₂-poor cortex or cerebellum. In peripheral organs, uptake peaked at 15 min but declined to baseline at 60 min, indicating good clearance from the body. In vitro autoradiography on transaxial and coronal brain sections showed specific binding of [³H]MCL-536, which was abolished by preincubation with D₂/D₃ ligands sulpiride, NPA, and raclopride and in the presence of the stable GTP analogue guanylylimidodiphosphate. In amphetamine-sensitized animals, striatal binding was higher than in controls, indicating specificity for the D₂^high receptor state. [³H]MCL-536's unique properties make it a valuable tool for research on neurological disorders involving the dopaminergic system like Parkinson's disease or schizophrenia.

Agonist high- and low-affinity states of dopamine D₂ receptors: methods of detection and clinical implications

Dopamine D(2) receptors, similar to other G-protein-coupled receptors, exist in a high- and low-affinity state for agonists. Based upon a review of the methods for detecting D(2) receptor agonist high-affinity states, we discuss alterations of such states in animal models of disease and the implications of such alterations for their labelling with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) tracers. The classic approach of detecting agonist high-affinity states compares agonist competition for antagonist radioligands, in most cases using [(3)H]-spiperone as the radioligand; alternative approaches and radioligands have been proposed, but their claimed advantages have not been substantiated by other investigators. In view of the advantages and disadvantages of various techniques, we critically have reviewed reported findings on the detection of D(2) receptor agonist high-affinity states in a variety of animal models. These data are compared to the less numerous findings from human in vivo studies based on PET and SPECT tracers; they are interpreted in light of the finding that D(2) receptor agonist high-affinity states under control conditions may differ between rodent and human brain. The potential advantages of agonist ligands in studies of pathophysiology and as diagnostics are being discussed.

Reward, addiction, withdrawal to nicotine

Nicotine is the principal addictive component that drives continued tobacco use despite users' knowledge of the harmful consequences. The initiation of addiction involves the mesocorticolimbic dopamine system, which contributes to the processing of rewarding sensory stimuli during the overall shaping of successful behaviors. Acting mainly through nicotinic receptors containing the α4 and β2 subunits, often in combination with the α6 subunit, nicotine increases the firing rate and the phasic bursts by midbrain dopamine neurons. Neuroadaptations arise during chronic exposure to nicotine, producing an altered brain condition that requires the continued presence of nicotine to be maintained. When nicotine is removed, a withdrawal syndrome develops. The expression of somatic withdrawal symptoms depends mainly on the α5, α2, and β4 (and likely α3) nicotinic subunits involving the epithalamic habenular complex and its targets. Thus, nicotine taps into diverse neural systems and an array of nicotinic acetylcholine receptor (nAChR) subtypes to influence reward, addiction, and withdrawal.

Dopamine D2 receptors as treatment targets in schizophrenia

The antipsychotic effectiveness of chlorpromazine and haloperidol started a search for their therapeutic targets. The antipsychotic receptor target turned out to be a dopamine receptor, now cloned as the dopamine D2 receptor. The D2 receptor is the common target for antipsychotics. Antipsychotic clinical doses correlate with their affinities for this receptor. Therapeutic doses of antipsychotics occupy 60 to 80% of brain D2 receptors in patients, but aripiprazole occupies up to 90%. While antipsychotics may take up to six hours to occupy D2 receptors, much clinical improvement occurs within a few days. The receptor has high- and low-affinity states. The D2High state is functional for dopamine-like agonists such as aripiprazole. Most individuals with schizophrenia are supersensitive to dopamine. Animal models of psychosis show that a variety of risk factors, genetic and nongenetic, are associated with behavioral supersensitivity to dopamine, reflected in elevated levels of dopamine D2High receptors. Although antipsychotics such as haloperidol alleviate psychosis and reverse the elevation of D2High receptors, long-term use of traditional antipsychotics can further enhance dopamine supersensitivity in patients. Therefore, switching from a traditional antipsychotic to an agonist antipsychotic such as aripiprazole can result in the emergence of psychotic signs and symptoms. Clozapine and quetiapine do not elicit parkinsonism and rarely result in tardive dyskinesia because they are released from D2 within 12 to 24 hours. Traditional antipsychotics remain attached to D2 receptors for days, preventing relapse, but allowing accumulation that can lead to tardive dyskinesia. Future goals include imaging D2High receptors and desensitizing them in early-stage psychosis.

In vitro and in vivo evidence for a lack of interaction with dopamine D2 receptors by the metabotropic glutamate 2/3 receptor agonists 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicaroxylate monohydrate (LY354740) and (-)-2-oxa-4-aminobicyclo[3.1.0] Hexane-4,6-dicarboxylic acid (LY379268)

Some recently published in vitro studies with two metabotropic glutamate 2/3 receptor (mGluR(2/3)) agonists [(-)-2-oxa-4-aminobicyclo[3.1.0] hexane-4,6-dicarboxylic acid (LY379268) and 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicaroxylate monohydrate (LY354740)] suggest that these compounds may also directly interact with dopamine (DA) D(2) receptors. The current in vitro and in vivo studies were undertaken to further explore this potential interaction with D(2) receptors. LY379268 and LY354740 failed to inhibit D(2) binding in both native striatal tissue homogenates and cloned receptors at concentrations up to 10 microM. LY379268 and LY354740 (up to 10 microM) also failed to stimulate [(35)S]GTPgammaS binding in D(2L)- and D(2S)-expressing clones in the presence of NaCl or N-methyl-d-glucamine. In an in vivo striatal D(2) receptor occupancy assay, LY379268 (3-30 mg/kg) or LY354740 (1-10 mg/kg) failed to displace raclopride (3 microg/kg i.v.), whereas aripiprazole (10-60 mg/kg) showed up to 90% striatal D(2) receptor occupancy. LY379268 (10 mg/kg) and raclopride (3 mg/kg) blocked d-amphetamine and phencyclidine (PCP)-induced hyperactivity in wild-type mice. However, the effects of LY379268 were lost in mGlu(2/3) receptor knockout mice. In DA D(2) receptor-deficient mice, LY379268 but not raclopride blocked both PCP and d-amphetamine-evoked hyperactivity. In the striatum and nucleus accumbens, LY379268 (3 and 10 mg/kg) was without effect on the DA synthesis rate in reserpinized rats and also failed to prevent S-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine-induced reductions in DA synthesis rate. Taken together, the current data fail to show evidence of direct DA D(2) receptor interactions of LY379268 and LY354740 in vitro or in vivo. Instead, these results provide further evidence for a novel antipsychotic mechanism of action for mGluR(2/3) agonists.

Dopamine D2(High) receptors on intact cells

In humans, behavioral dopamine supersensitivity occurs in schizophrenia and in Parkinson's disease. In animals, behavioral dopamine supersensitivity is consistently associated with increased dopamine D2(High) receptors in homogenized striata in vitro. Because D2(High) receptors have not yet been detected in intact cells, we used [(3)H]domperidone to detect D2(High) sites in intact rat anterior pituitary adenoma culture cells. Although [(3)H]raclopride and [(3)H]spiperone did not detect D2(High) receptors in intact cells or in rat fresh striatal slices, [(3)H]domperidone readily detected D2(High) receptors, warranting an in vivo search for D2(High) variations in human diseases.

Dopamine D2(High) receptors moderately elevated by sertindole

Because long-term administration of antipsychotics can cause behavioral dopamine supersensitivity, this study examined whether the antipsychotic sertindole could elicit biochemical changes indicative of dopamine supersensitivity. In rats, behavioral dopamine supersensitivity is consistently associated with an increased proportion of dopamine receptors that have high affinity for dopamine, namely D2(High), in homogenized striata. Nine days of subcutaneously injected sertindole (1.25 mg/kg/day) increased the proportion of D2(High) receptors between 186% and 215%, although the total population of D2 receptors did not change. Although the findings suggest that rats or patients treated with sertindole might exhibit behavioral dopamine supersensitivity, the drug-induced increase in D2(High) receptors was less than that previously found with haloperidol.

Antiparkinson concentrations of pramipexole and PHNO occupy dopamine D2(high) and D3(high) receptors

Because the high-affinity state of dopamine D2 receptors, D2(High), is the functional state of D2, and because the proportion of D2 receptors in the high-affinity state correlates with dopamine behavioral supersensitivity, the present study was designed to determine the affinities of antiparkinson dopamine agonists at the D2(High) site by means of competition with [3H]domperidone. In contrast to [125I]iodosulpride or [3H]spiperone, which are not sensitive to low concentrations of dopamine agonists, [3H]domperidone readily reveals dissociation constants (K(i)) for antiparkinson agonists at D2(High) and D3(High) receptors. The K(i) values for the human cloned D2(High) and D3(High) receptors, respectively, were 19 and 9 nM for pramipexole, 0.24 and 0.6 nM for +PHNO, 0.7 and 1.3 nM for bromocriptine, 0.5 and 2.6 nM for apomorphine, and 0.09 and 0.25 nM for (-)N-propylnorapomorphine. After correcting for the fraction of drug bound to plasma proteins, the known clinical concentrations in plasma or plasma water of these drugs, including pramipexole and +PHNO, are sufficient to occupy and activate the high-affinity state of D2, D2(High), in treating Parkinson's disease. The D3(High) receptors are less selectively occupied by +PHNO, bromocriptine, apomorphine, and -NPA.

Dopamine displaces [3H]domperidone from high-affinity sites of the dopamine D2 receptor, but not [3H]raclopride or [3H]spiperone in isotonic medium: Implications for human positron emission tomography

Because the high-affinity state of the dopamine D2 receptor, D2High, is the functional state of the receptor, has a role in demarcating typical from atypical antipsychotics, and is markedly elevated in amphetamine-sensitized rats, it is important to have a method for the convenient detection of this state by a ligand. The present data show that, in contrast to [(3)H]spiperone or [(3)H]raclopride, [(3)H]domperidone labels D2High sites in the presence of isotonic NaCl in either striatum or cloned D2Long receptors, yielding a dopamine dissociation constant (1.75 nM) in agreement with that found with [(3)H]dopamine. Increased labeling of D2High sites occurred with [(3)H]domperidone after severe disruption of the cells, suggesting that [(3)H]domperidone has better access to the D2 receptor from the cytoplasmic aspect of the cell membrane. The density of the [(3)H]domperidone-labeled D2 receptors was the same as that of the [(3)H]raclopride-labeled D2 receptors, but twice the density of [(3)H]spiperone sites for human cloned D2Long receptors, compatible with the monomer-dimer concept of the D2 receptor. [(3)H]domperidone readily labels the D2High sites in postmortem human brain homogenates. Although [(3)H]spiperone or [(3)H]raclopride can occupy D2High sites, the inability of 1-10 nM dopamine to displace these ligands under isotonic conditions suggests that these ligands may not be suitable for monitoring the physiological high-affinity state of the dopamine D2 receptor by means of [(11)C]methylspiperone or [(11)C]raclopride in humans.

The human dopamine D2(Longer) receptor has a high-affinity state and inhibits adenylyl cyclase

Brain dopamine D2 receptors are the main targets for antipsychotic and anti-Parkinsonian drugs. The dopamine D2 receptor has three forms, D2(Short), D2(Long) and D2(Longer). D2(Longer) is a newly found splice variant which contains two additional amino acids (valine and glutamine) in the third cytoplasmic loop of the receptor. To determine whether D2(Longer) was functional, the cDNA was transfected into CHO cells. D2(Longer) revealed a high-affinity state for dopamine ( approximately 1.5 nM), and mediated dopamine-inhibited adenylyl cyclase.

Comparative biochemical pharmacology of central nervous system dopamine D1 and D2 receptors

The biochemical properties of central nervous system (CNS) dopamine (DA) D1 and D2 receptors were examined using the specific antagonists [3H]SCH23390 and [3H]raclopride, respectively. There is a different participation of sulfhydryl (-SH) and disulfide (-SS-) groups in the binding site and/or coupling to second messenger systems of D1 and D2 receptors. The ionic studies with [3H]SCH23390 showed slight agonist and antagonist affinity shifts for the D1 receptor. On the other hand, the D2 receptor is very sensitive to cations; even if lithium and sodium influence specific [3H]raclopride binding in a similar manner, there appear to be quantitative differences between these two ions that cannot be explained by surface charge mechanisms. The distribution of D1 and D2 receptors was heterogenous in both species, with the greatest densities in the neostriatum, where the highest concentrations of DA and metabolites were measured. Regions with low endogenous DA content (cerebral cortex and hippocampus) had lower densities of DA receptors. Furthermore, these binding sites were differentially localized within the various regions, and there were substantially more D1 than D2 receptors. The functional significance and heterogeneities in the distribution of D1 and D2 receptors can be related to dopaminergic innervation and turnover.

Effects of monovalent cations on neostriatal dopamine D2 receptors labeled with [3H]raclopride

Specific [3H]raclopride binding to dopamine D2 receptors in the rabbit neostriatum was investigated in the presence of the monovalent cations sodium, lithium and potassium. NaCl and LiCl produced concentration-dependent elevations in specific [3H]raclopride binding with sodium inducing approximately 50% more binding than lithium. Inhibition of [3H]raclopride binding by the antagonist (+)-butaclamol was unaffected by the presence of sodium or lithium in the incubation medium. In contrast, the potency of dopamine to compete with [3H]raclopride was decreased by these two ions. This effect was more pronounced in the presence of sodium than lithium and was observed for both the high- and low-affinity states of the D2 receptor. The guanine nucleotide derivative 5'-guanylylimidodiphosphate (Gpp(NH)p) reduced the potency of dopamine to compete with [3H]raclopride binding in both the presence and absence of cations; however, this effect of Gpp(NH)p was a shift of the D2 receptors from a high to a lower affinity state. Saturation binding curves in the presence of sodium or lithium were compared with experiments carried out in the absence of monovalent cations (sucrose) and demonstrated that these ions increased the affinity (judged by the equilibrium dissociation constant Kd) of the neostriatal [3H]raclopride binding sites. While NaCl produced a significantly greater change in the Kd of [3H]raclopride binding as compared to LiCl, no differences were apparent in the maximum binding capacity (Bmax) values determined in the presence of these two cations. In conclusion, the results indicate that [3H]raclopride binding to rabbit neostriatal membranes exhibits a sensitivity to monovalent cations that is consistent with the ionic regulatory properties of the D2 receptor. Moreover, although lithium and sodium influence specific [3H]raclopride binding in a similar manner, there appear to be quantitative differences between these two ions.

Cloning of the cDNA and gene for a human D2 dopamine receptor

A clone encoding a human D2 dopamine receptor was isolated from a pituitary cDNA library and sequenced. The deduced protein sequence is 96% identical with that of the cloned rat receptor with one major difference: the human receptor contains an additional 29 amino acids in its putative third cytoplasmic loop. Southern blotting demonstrated the presence of only one human D2 receptor gene. Two overlapping phage containing the gene were isolated and characterized. DNA sequence analysis of these clones showed that the coding sequence is interrupted by six introns and that the additional amino acids present in the human pituitary receptor are encoded by a single exon of 87 base pairs. The involvement of this sequence in alternative splicing and its biological significance are discussed.

Mono- and divalent cations modulate the affinities of brain D1 and D2 receptors for dopamine by a mechanism independent of receptor coupling to guanyl nucleotide binding proteins

In order to clarify the question of whether the modulatory effects of cations on dopamine receptor affinities are brought about by shifts in the equilibrium of receptor - G protein - coupling, it was investigated whether mono- and divalent cations were still able to modulate rat striatal D1 and D2 receptor affinities after selective inactivation of the G-proteins linked to the two receptors. The Gs-protein coupled to the D1 receptor was eliminated by mild thermal inactivation, and the Gi- (or Go-) protein associated with the D2 receptor by alkylation with a low concentration of N-ethyl-maleimide. Incubation of striatal membranes at 60 degrees C completely abolished the specific binding of 3H-GTP. Both treatments resulted in an increase of the IC50-values for dopamine as a displacer of 3H-SCH 23390 from D1- and of 3H-spiperone from D2 receptors. Concomitantly, the formerly shallow D1 displacement curves became steeper, with their Hill coefficients increasing. This effect was less evident at D2 receptors. Guanosine triphosphate (GTP), which increased the IC50's of dopamine for both receptors approximately two-fold in control membranes, was without effect in pretreated samples, indicating an effective inactivation of the G-proteins. Na+ ions were still able to lower, and Ca2+ ions to increase the affinities of D1 and D2 receptors for dopamine after such inactivation of the respective G-proteins. It is concluded that the mechanism underlying the regulation of dopamine receptor affinities by mono- and divalent cations is independent of and superimposed upon the coupling of these receptors to guanyl nucleotide binding proteins.

D2 dopamine receptors in rat striatum are homogeneous as revealed by ligand-binding studies

D2 dopamine receptors in rat striatum have been analysed using the binding of [3H]domperidone. Competition experiments were performed with classical dopamine antagonists and antagonists reported by other workers to discriminate D2 dopamine receptor sub-classes. In all cases competition data conformed to a single binding site interaction so that there is no evidence for heterogeneity of the D2 dopamine receptor.

Heterogeneity of D2 dopamine receptors in different brain regions

The binding of [3H]spiperone has been examined in membranes derived from different regions of bovine brain. In caudate nucleus, nucleus accumbens, olfactory tubercle and putamen binding is to D2 dopamine and 5HT2 serotonin receptors, whereas in cingulate cortex only serotonin 5HT2 receptor binding can be detected. D2 dopamine receptors were examined in detail in caudate nucleus, olfactory tubercle and putamen using [3H]spiperone binding in the presence of 0.3 microM-mianserin (to block 5HT2 serotonin receptors). No evidence for heterogeneity among D2 dopamine receptors either between brain regions or within a brain region was found from the displacements of [3H]spiperone binding by a range of antagonists, including dibenzazepines and substituted benzamides. Regulation of agonist binding by guanine nucleotides did, however, differ between regions. In caudate nucleus a population of agonist binding sites appeared resistant to guanine nucleotide regulation, whereas this was not the case in olfactory tubercle and putamen.

Dopamine receptors and the dopamine hypothesis of schizophrenia

The discovery of neuroleptic drugs in 1952 provided a new strategy for seeking a biological basis of schizophrenia. This entailed a search for a primary site of neuroleptic action. The Parkinsonian effects caused by neuroleptics suggested that dopamine transmission may be disrupted by these drugs. In 1963 it was proposed that neuroleptics blocked "monoamine receptors" or impeded the release of monoamine metabolites. The neuroleptic concentration in plasma water or cerebrospinal fluid was of the order of 2 nM for haloperidol in clinical therapy. A systematic research was made between 1963 and 1974 for a primary site of neuroleptic action which would be sensitive to 2 nM haloperidol and stereoselective for (+)-butaclamol. Direct evidence that neuroleptics selectively blocked dopamine receptors occurred in 1974 with the finding that nanomolar concentrations of these drugs stereoselectively inhibited the binding of [3H]-dopamine or [3H]-haloperidol. These binding sites, now termed D2 dopamine receptors (which inhibit adenylate cyclase), are blocked by neuroleptics in direct relation to the antipsychotic potencies of the neuroleptics. No such correlation exists for D1 receptors (which stimulate adenylate cyclase). Based on the fact that dopamine-mimetic drugs elicited hallucinations, and that neuroleptics caused rigidity, Van Rossum in 1966 had suggested a hypothesis that dopamine pathways may be overactive in schizophrenia. The D2-selective blockade by all neuroleptics (except the monoamine-depleting reserpine) provided strong support for the dopamine hypothesis. Further support now comes from postmortem data and in vivo positron tomographic data, both of which indicate that the density of D2 receptors are elevated in the schizophrenic brain. The postmortem data indicate a bimodal pattern with half the schizophrenics having striatal D2 densities of 14 pmol/g (control is 13 pmol/g) and the other half having 26 pmol/g. Current positron tomographic data indicate D2 densities of 14 pmol/g in control subjects, but values of 34 pmol/g in drug-naive schizophrenics. Future tests of the dopamine hypothesis of schizophrenia may entail an examination of the amino acid composition and genes for D2 receptors in schizophrenic tissue, an examination of the ability of the D2 receptor to become phosphorylated and to desensitize into the low-affinity state, and an examination of the interaction of D2 receptors with D1 receptors or other neurotransmitters.